1. Field of the Invention
The present invention relates to a control apparatus that uses an alcohol fuel to be supplied to an internal combustion engine.
2. Description of the Related Art
Recently, with air pollution, changes in the oil situation, and the like, alcohol has attracted attention as an alternate fuel, which is to be supplied to an internal combustion engine (hereinafter, referred to as an “engine”). For example, a flexible fuel vehicle (FFV) using a blend fuel of alcohol and gasoline (at an alcohol concentration of 0% to 100%) has been put into practical use. However, since a theoretical air-fuel ratio (weight ratio of air and a fuel necessary for complete combustion of the fuel) of alcohol differs depending on its concentration, the amount of the fuel according to the alcohol concentration must be injected.
In order to inject the amount of the fuel according to the alcohol concentration, a fuel injection amount obtained by a conventional computation is generally corrected with a correction amount according to the alcohol concentration. For example, when a level of the amount of the fuel in a fuel tank is varied by a predetermined value or larger, an error in the air-fuel ratio based on an output from an oxygen sensor provided to an exhaust pipe is detected. The alcohol concentration is estimated according to the detected error. Then, the fuel injection amount is corrected according to the estimated alcohol concentration (for example, see U.S. Pat. No. 6,016,796).
On the other hand, in the engine provided with a plurality of cylinders, the oxygen sensor for detecting the air-fuel ratio of an exhaust gas is provided in an assembly portion of the exhaust pipes of the respective cylinders to detect an average air-fuel ratio of all the cylinders. Air-fuel ratio feedback control is performed to allow the average air-fuel ratio to be equal to a target air-fuel ratio.
However, the air-fuel ratio in each individual cylinder is not necessarily the target air-fuel ratio due to a variation in injection characteristics of an injector for supplying a fuel to each of the cylinders, time degradation caused by clogging of a nozzle hole of the injector, or the like. Thus, there is a variation in the air-fuel ratio between the cylinders, failing to supply an appropriate fuel injection amount to each of the cylinders in some cases. As a result, the combustion and the exhaust gas sometimes deteriorate. Therefore, the diagnosis of an injector failure, which is caused by the variation in the air-fuel ratio between the cylinders, is required.
For example, the number of times (number of inversions) at which an output value from the oxygen sensor provided in the assembly portion of the exhaust pipes, which is detected for each predetermined cycle, passes through a predetermined threshold value is counted. When the number of inversions within a predetermined period of time is equal to or larger than a predetermined number, it is determined that the output value from the oxygen sensor has a large pulsation (fluctuation), specifically, a variation occurs in the air-fuel ratio in a combustion stroke of each of the cylinders to make a diagnosis of the occurrence of a failure of the injector (for example, see Japanese Examined Patent Publication No. H07-009201).
The variation in the air-fuel ratio between the cylinders also occurs when a fuel having characteristics (volatility, theoretical air-fuel ratio, or the like) which are different from those of the currently used fuel is fed. For example, the variation in the air-fuel ratio occurs in the following case and the like. A returnless system including no flow path from the fuel pump to four injectors, through which the fuel returns to the fuel tank, is used as a fuel supply system. The fuel is switched from gasoline at the alcohol concentration of 0% (E0) to the alcohol fuel at the alcohol concentration of 100% (E100) to be fed.
Since the gasoline still remains in the fuel pipe and a delivery pipe immediately after the alcohol fuel is fed, the gasoline is supplied to the injector of each of the cylinders immediately after engine start. Then, with the consumption of the remaining gasoline for an engine operation, the alcohol fuel moves in the fuel pipe. As a result, the alcohol fuel reaches the delivery pipe. Here, since a sectional area of the delivery pipe is larger than that of the fuel pipe, the gasoline and the alcohol fuel mix with each other in the delivery pipe to be distributed to the injectors. Therefore, with the consumption of the fuel, the fuel changes from the fuel at a low alcohol concentration close to that of the gasoline at the beginning to the fuel at a high alcohol concentration close to that of the alcohol fuel. Finally, only the alcohol fuel reaches the injectors. However, since the concentration starts changing from the inlet side of the delivery pipe because of a shape of the delivery pipe, the fuel at the same concentration cannot be supplied to all the injectors at the same timing. Specifically, although the alcohol concentration starts changing at an early point in time in the cylinder close to the inlet of the delivery pipe, the alcohol concentration starts changing at the latest point in time in the cylinder on the remote side of the delivery pipe.
Since the variation occurs in the timing of occurrence of the change in the concentration of alcohol supplied to the injectors, a variation also occurs in the air-fuel ratio between the cylinders. An alcohol concentration value used in the control is generally detected by the oxygen sensor provided in the assembly portion of the exhaust pipes of the respective cylinders or an alcohol concentration sensor provided to the fuel pipe. Further, the fuel injection amount is corrected according to the detected alcohol concentration. However, a difference in the change in the alcohol concentration in the injector between the cylinders cannot be detected. As the (estimated) alcohol concentration value, an average alcohol concentration of the first to fourth cylinders is detected. As a result of the fuel injection amount control using the average alcohol concentration value, the fuel injection amount becomes lean in the fourth and third cylinders with respect to the target air-fuel ratio, whereas the fuel injection amount becomes rich in the first and second cylinders with respect to the air-fuel ratio. As a result, the variation temporarily occurs in the air-fuel ratio between the cylinders.
Moreover, since a fluctuation in the oxygen sensor output becomes large due to the variation in the air-fuel ratio between the cylinders, the air-fuel ratio control (fuel injection amount control) cannot be accurately controlled by using the output value from the oxygen sensor, resulting in the deterioration of the combustion and the exhaust gas of each of the cylinders.
Further, according to the conventional methods, an accurate failure diagnosis can be made for a lasting variation such as the variation in injection characteristics between the injectors or the time degradation due to the clogging of the nozzle hole of the injector in the diagnosis of the failure of the injector based on the variation in the air-fuel ratio between the cylinders. However, the temporary variation in the air-fuel ratio occurring when the alcohol concentration changes is not assumed. Therefore, as a result of the detection of the fluctuation in the oxygen sensor output value when the alcohol concentration changes, there is a fear that a wrong diagnosis of the occurrence of the failure may be made although the injector is normal. Thus, there is a problem that the combustion and the exhaust gas are deteriorated.